1. Field of the Invention
The present invention concerns circuits for use in gain control. The gain control of signals is used in many applications and one field of use is in controlling amplification or attenuation in the line cards in a telephone exchange. In particular the gain control circuit with which the present invention is concerned enables the value of attenuation or amplification to be controlled by digital values.
2. Description of Related Art
One currently used type of gain control circuit which will be described in greater detail hereinafter utilises a network of resistors arranged in what is known as the R-2R- ladder principle in combination with an operational amplifier. In this known circuit an input current is split by the resistor network into two paths which are held at 0 volts. The ratio of current in the two paths is dependent on the way in which the network is switched.
This known system has problems in providing adequate range combined with adequate resolution. Effectively good resolution can be obtained over a limited range, but any attempt to increase the range leads to unacceptable loss of resolution.